A.) Field of Use
The present invention relates to articles and methods that are useful for treating exhaust gases generated during hydrocarbon combustion. More particularly, the invention relates to catalytic filters for reducing NOx and soot in exhaust gas streams, such as those generated by diesel engines.
B.) Description of Related Art
Exhaust gas produced by on-road vehicles in the United States currently contributes about a third of the country's smog-producing air pollution. Efforts to reduce smog include the use of more fuel efficient engines, such as diesel engines compared to gasoline engines, and improved exhaust gas treatment systems.
The largest portions of most combustion exhaust gases contain relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2); but the exhaust gas also contains in relatively small part noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). One of the most burdensome components of vehicular exhaust gas is NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). The production of NOx is particularly problematic for lean burn engines, such as diesel engines. To mitigate the environmental impact of NOx in exhaust gas, it is desirable to eliminate these undesirable components, preferably by a process that does not generate other noxious or toxic substances
The exhaust gas of diesel engines tends to have more soot compared to gasoline engines. Soot emissions can be remedied by passing the soot-containing exhaust gas through a particulate filter. However, the accumulation of soot particles on the filter can cause an undesirable increase in the back pressure of the exhaust system during operation, thereby decreasing efficiency. To regenerate the filter, the accumulated carbon-based soot must be removed from the filter, for example by periodically combusting the soot by passive or active oxidation. One such technique involves catalytic oxidation of the soot at low temperatures. For example, U.S. Pat. No. 4,902,487 teaches the use of NO2 as an oxidant serving to effectively combust the collected soot at low temperatures. It has also been suggested that performance of a catalytic soot filter can be improved by overlapping different oxidation catalysts on a wall-flow soot filter (US Pat. Pub. No. 2009/0137386) or by zoning the oxidation catalyst using different catalyst concentrations (EP Pat. No. 1 859 864).
For lean burn exhaust gas, such as diesel exhaust gas, reducing reactions are generally difficult to achieve. However, one method for converting NOx in a diesel exhaust gas into more benign substances is commonly referred to as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a reducing agent, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant, typically anhydrous ammonia, aqueous ammonia, or urea, is added to an exhaust gas stream prior to contacting the catalyst. The reductant is absorbed onto a catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for a stoichiometric reaction using either anhydrous or aqueous ammonia for an SCR process is:4NO+4NH3+3O2→4N2+6H2O2NO2+4NH3+3O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Known SCR catalysts include zeolites or other molecular sieves disposed on or in a monolithic substrate. Examples of such molecular sieves include materials having a chabazite framework (e.g., SSZ-13 and SAPO-34), beta framework, mordenite framework (e.g., ZSM-5), and the like. To improve the material's catalytic performance and hydrothermal stability, molecular sieves for SCR applications often include exchanged metal ions that are loosely held to the molecular sieve's framework.
Since SCR catalysts generally serve as heterogeneous catalysts (i.e., solid catalyst in contact with a gas and/or liquid reactant), the catalysts are usually supported by a substrate. Preferred substrates for use in mobile applications include flow-through monoliths having a so-called honeycomb geometry that comprises multiple adjacent, parallel channels that are open on both ends and generally extend from the inlet face to the outlet face of the substrate. Each channel typically has a square, round, hexagonal, or triangular cross-sectional. Catalytic material is applied to the substrate typically as a washcoat or other slurry that can be embodied on and/or in the walls of the substrate.
Exhaust systems containing multiple components, even multiple SCR catalysts, are known. For example, U.S. Pat. No. 7,767,176 describes an exhaust system having two substrates, preferably non-filtering flow-through honeycombs, arranged in series wherein each substrate contains an SCR catalyst. Zoning non-filtering flow-through substrates with SCR catalysts followed by oxidation catalysts is also known (e.g., U.S. Pat. No. 5,516,497).
To reduce the amount of space required for an exhaust system, it is desirable to design individual exhaust components to perform more than one function. For example, applying an SCR catalyst to a wall-flow filter substrate instead of a flow-through substrate serves to reduce the overall size of an exhaust treatment system by allowing one substrate to serve two functions, namely catalytic conversion of NOx by the SCR catalyst and removal of soot by the filter. US Pat. Pub. 2010/0180580 discloses an SCR catalyst can be applied to a honeycomb substrate in the form of a wall-flow filter. Wall-flow filters are similar to flow-through honeycomb substrates in that they contain a plurality of adjacent, parallel channels. However, the channels of flow-through honeycomb substrates are open at both ends, whereas the channels of wall-flow substrates have one end capped, wherein the capping occurs on opposite ends of adjacent channels in an alternating pattern. Capping alternating ends of channels prevents the gas entering the inlet face of the substrate from flowing straight through the channel and existing. Instead, the exhaust gas enters the front of the substrate and travels into about half of the channels where it is forced through the channel walls prior to entering the second half of the channels and exiting the back face of the substrate.
A wall-flow filter having an SCR (SCRF) and an oxidation catalyst, wherein the SCR catalyst is disposed upstream of an oxidation catalyst, is described in GB Pat. Appln. 1003784.4, which is incorporated herein in its entirety by reference. However, there remains the need for improved SCRF systems having good catalytic performance while also having minimal back pressure.